This invention is intended to present a blue laser device of the similar method as the conventional infrared and red laser diode, by introducing a method based on a new principle in the method of exciting light emitting devices such as semiconductor laser highly valuable in the field of optoelectronics, and further optimizing the construction.
The blue laser beam is keenly demanded in the field where laser beams of short wavelength are desired such as information processing and measurements, and is strongly needed to be developed as early as possible. However, the materials capable of emitting blue laser beams are limited to only a part of crystal systems large in band gaps, and in order to obtain laser diodes by the conventional current injection by using such materials, the crystal growth technology is not sufficient yet. It is also proposed to obtain blue laser beams by using harmonics of long wavelength, but the output is small, which is not suited to integration structurally. The invention is intended to present a blue laser device of high performance on the basis of new principle and construction by eliminating the difficulties of the prior art, and its industrial value of use is extremely high.
Various excitation methods of laser device are known and realized. In the solid laser and liquid laser, light of an intense flash lamp or auxiliary laser device is emitted to form an inversion distribution (optical pumping). In the gas laser, a plasma state is produced by application of a high voltage, and gas is excited by mutual collision of atoms, ions and electrons in the plasma. In most semiconductor lasers, an electric current is injected into the p-n junction to form an inversion distribution. Very rarely, if the p-n junction is not formed favorably, optical pumping or electron beam excitation have also been attempted in semiconductor lasers.
In the conventional method of electron beam excitation, electrons released from an ordinary hot cathode are accelerated to an adequate energy (100 eV to 10 keV) to irradiate the active layer, and therefore the device size is large, and the heat of the hot cathode raises the temperature of the active layer, and these adverse effects are unavoidable. That is, the conventional attempts of optical pumping of semiconductor laser and electron beam excitation were the methods used as the means for evaluating the properties of the semiconductor crystals serving as the active layers, and were not the methods for allowing the devices to act practically as useful devices.
Thus, in almost all conventional semiconductor lasers, p-type and n-type clad layers were provided, and the active layer was placed between them. Such a structure is excellent for efficient excitation by current injection, but it cannot be employed without the preliminary condition that p-type and n-type crystals of high quality be obtained easily. Such a condition is satisfied by GaAs or InP crystals, and the laser diodes useful in the wavelength regions of infrared and red light are practical. On the other hand, SiC or ZnSe-ZnS crystals having band gaps corresponding to the blue wavelength region involve intrinsic difficulties in crystal growth, and in spite of efforts in development for over 10 years, free conductivity control has not yet been achieved at present, and the future outlook is not necessarily bright. Therefore, considering the extension of the conventional structure, the possibility of the realization of a blue semiconductor laser device is, in fact, extremely small. Incidentally, in the excitation method conventionally employed in the solid laser or gas laser, it is impossible, in principle, to reduce the size to that of an ordinary semiconductor laser.
Vacuum sealing of a space in a micrometer size has not been experienced in the past. The size of a conventional vacuum tube was larger by many orders of magnitudes, and although voids on the order of a micrometer might be formed in the refining process of metal, they were not formed intentionally by man. But by employing the modern sub-micron processing technology advanced mainly from the demands of Si technology, the past dream has come to be realized.
Lasers to emit blue light include Ar and other gas laser, solid lasers, excimer laser, and dye laser. They are all commercially available, and are used in measurement or auxiliary means for film deposition, but in these lasers, according to their principle, it is difficult to reduce the size. This is because the atoms and molecules as the source of emission are scattered about sporadically in space, and a considerably large volume (total number of emission sources) must be required in order to obtain a threshold emission intensity to reach the laser oscillation. Besides, the method of excitation involves other problems. In order to form a plasma state, a high voltage of tens of keV must be applied, and in order to withstand such a voltage, the electrode interval naturally becomes wider, which is contradictory to reduction of size. As a construction completely different in principle from such gas lasers, a second harmonic generator (SHG) is known, and it is noticed as a blue laser beam source as small as an ordinary semiconductor laser. The SGH is, by making use of the nonlinear characteristics of lithium niobate (LiNbO.sub.3) or the like, intended to convert the output of an ordinary semiconductor laser releasing infrared rays into harmonics of doubled frequency and to output them. The SHG is used as a blue light source, but, as its demerits, the size is slightly larger than that of a semiconductor laser, and the output light is, in the Cerenkov radiation type, delivered at a certain angle to the plane including the input light and waveguide so that it is not suited to integration with other devices, and the conversion efficiency into harmonics is small and hence the output is small.